[tsubaki]

Not knowing any of the motor's past history will leave it open to a lot of stuff.
Are you sure the timing is correct (the fuel linkage to spark advance ratio), I had (got ) a 70 horse that does that when fully warmed up and am suspicious that this is the problem.
How do the plugs look, is it possible one is fouled or not as clean as it should be?
Coils and plug wires new or at least not cracked? One way to check is to crank it up and scoot it with a garden hose to see if it dies, if not it is probably alright.
Confident the fuel pump is correct in delivery? While we are there what oil system do you have?
Yes it sounds like a fuel restriction or a kinked hose or a clogged filter or a dozen other things.
Carb cleaning is only as good as the "mechanic" doing it, question that portion also.
All in all, a little history will hopefully narrow it down.
WELCOME TO THE SITE! Where are the pictures??